The present invention relates to a semiconductor device and, more particularly, to a semiconductor device using a MIS (Metal Insulator Semiconductor) capacitor which prevents malfunction due to .alpha.-rays radiated from a slight amount of radioactive material contained in a packaging material or the like.
Many conventional circuits constituted by a transistor have a structure such as that shown in FIG. 9. Such a circuit is called an emitter follower circuit in which an input V.sub.B is connected to the base of a transistor 11 and the emitter constitutes an output V.sub.OUT of the circuit. The emitter and the collector of a transistor 91 inserted between the output V.sub.OUT and the power source V.sub.EE are connected to the output V.sub.OUT, and the base of the transistor 91 is connected to the power source V.sub.EE. The transistor 91 having the above-described structure works as a junction capacitance means and has a function of stabilizing the output potential. FIGS. 10a and 10b show an example of circuit patterns for realizing the circuit shown in FIG. 9 in a semiconductor device, wherein FIG. 10a is a plan view and FIG. 10b is a sectional view thereof. In FIG. 10a, a wiring material such as Al is used for the connection between the input V.sub.B and the base electrode 22 of the transistor 11, the connection between the emitter electrode 21 of the transistor 11 and the electrode 107 of a resistor 15, and the emitter electrode 101 and the collector electrode 103 of the transistor 91 which transistor functions as a capacitance means. FIG. 10b is a sectional view of the circuit pattern shown in FIG. 10a, taken along the line D - D', showing the sectional structure and the connected state of the resistor 15 and the transistor 91 provided on a p-substrate 10. In FIGS. 9, 10a and 10b, the symbol Vcc represents a ground potential, the reference numeral 23 a collector electrode, 73 an insulator, 102 a base electrode, 104 an n-type collector layer, 105 a p-type base layer and 106 an n-type emitter layer.
As described above, as a capacitance means for stabilization of a potential, using a junction capacitance means, such as a transistor is typically conventional often used. However, a capacitor means having a structure such as that shown in FIG. 7 has recently come to be used. Such a capacitor means is generally called a MIS capacitor, and is composed of an N.sup.+ (or P.sup.+) buried layer 72 provided on the p- (or n-) substrate 10, a dielectric-side electrode 13 lead from the buried layer 72 through a dielectric 71, and a buried layer-side electrode 14 connected to the buried layer 72. The electrodes 13 and 14 are symbolically shown in FIG. 7. Since a MIS capacitor has a greater capacitance value per unit area than a junction capacitance means, it is possible to reduce the layout area of the MIS capacitor. Such a MIS capacitor will therefore be frequently used hereafter. A method directed to the fabrication of a MIS capacitor is disclosed in, for example, Japanese Patent Application Laid-Open No. 111459/1987, published on May 22, 1997 (Hilachi Ltds). In this prior art, however, an influence resulting from external noise and, in particular, influence from .alpha.-ray radiation is not taken into consideration. In FIG. 9, when an .alpha.-ray enters the transistor 91 constituting a junction capacitance means, a noise current due to the .alpha.-ray flows from the collector to the substrate of the integrated circuit, as described in Japanese Patent Application Laid-Open No. 169015/1986, published on July 30, 1986, (Hitachi, Ltd.) so that the potential of the output V.sub.OUT is temporarily greatly lowered from the set value. Even in the case of using a MIS capacitor such as that shown in FIG. 7 in place of the junction capacitance means, the potential of the output V.sub.OUT is subjected to an influence of an .alpha.-ray or the like. The reason for this will be described hereinunder.
FIG. 8 shows an equivalent circuit of the MIS capacitor shown in FIG. 7. In FIG. 8, a MIS capacitor 12 is a capacitance means provided between the N.sup.+ buried layer 72 and the dielectric-side electrode 13 shown in FIG. 7, and a diode 81 is composed of the N.sup.+ buried layer 72 and the p-substrate 10. In the case of using such a capacitance means, since there is no polarity relationship between the dielectric-side electrode 13 and the buried layer-side electrode 14 unlike that of the junction capacitance means, either electrode may be selected with respect to a certain connection node (e.g., the output V.sub.OUT in FIG. 9). However, since the capacitance value changes in accordance with the selected electrode and the fact that the capacitance value becomes larger, for example, when the buried layer-side electrode 14 is connected to the output V.sub.OUT in FIG. 9, such connection is generally employed. This is because the junction capacitance due to the diode 81 is added. The reference numeral 82 denotes a substrate-side electrode.
When an .alpha.-ray enters the MIS capacitor having the above-described connection, the electrons generated within the substrate flow to the N.sup.+ buried layer 72, so that the output V.sub.OUT is temporarily greatly reduced in the same way as in the case of using the junction capacitance means. As a result, because of the negative pulse noise effected, a malfunction is on a circuit using the output V.sub.OUT. The negative pulse noise corresponds to a pulse noise potentially directed toward the negative side.